1. Field of the Invention
The present invention relates to optical substrates having a structured surface, particularly to optical substrates for brightness enhancement, and more particularly to brightness enhancement substrates for use in flat panel displays having a planar light source.
2. Description of Related Art
Flat panel display technology is commonly used in television displays, computer displays, and handheld electronics (e.g., cellular phones, personal digital assistants (PDAs), etc.). Liquid crystal display (LCD) is a type of flat panel display, which deploys a liquid crystal (LC) module having an array of pixels to render an image. In backlight LCDs, brightness enhancement films use prismatic structures to direct light along the viewing axes (i.e., normal to the display), which enhances the brightness of the light viewed by the user of the display and which allows the system to use less power to create a desired level of on-axis illumination.
Heretofore, brightness enhancement films were provided with parallel prismatic grooves, lenticular grooves, or pyramids on the light emitting surface of the films, which change the angle of the film/air interface for light rays exiting the films and cause light incident obliquely at the other surface of the films to be redistributed in a direction more normal to the exit surface of the films. The brightness enhancement films have a light input surface that is smooth, through which light enters from the backlight module. Heretofore, many applications used two brightness enhancement film layers rotated relative to each other such that the grooves in the respective film layers are at 90 degrees relative to each other.
An undesirable effect arising from using two brightness enhancement films in a flat panel display is the appearance of moiré patterns caused by the interference of the two periodic patterns of the prismatic structures on the surfaces of the two brightness enhancement films. In the past, brightness enhancement films have been developed with various surface structural configurations in an attempt to avoid moiré pattern formation. In a flat panel display that incorporates a single layer of brightness enhancement film, the periodic patterns causing moiré are the patterns of the prismatic structure on the film itself and the reflected image of such patterns (as reflected by other surfaces in the flat panel display). Further, the structures on the brightness enhancement film and the pixel array in the LC module could create moiré patterns as well.
For example, U.S. Pat. No. 5,280,371 discloses the use of different spatial frequencies or pitches of parallel for the two layers of brightness enhancement films. Further, it discloses rotating at least one of the brightness enhancement films with respect to the pixel array in the LC module such that the longitudinal structures on the film is at an angle to the pixel array to reduce moiré effect. However, due to conventional manufacturing processes for brightness enhancement films, significant trimming is required to obtain a rectangular shaped brightness enhancement film for use with a rectangular flat panel display, such that the prismatic structures are rotated at an angle relative to the pixel array in the LC module. This significantly increases costs of production.
U.S. Pat. No. 5,919,551 discloses a structured optical film with parallel, variable pitch peaks and/or grooves to reduce the visibility of moiré interference patterns and optical displays incorporating one or more layers of the film. The pitch variations can be over groups of adjacent peaks and/or valleys or between adjacent pairs of peaks and/or valleys. The cross sectional views across the optical film remain constant along the peaks and valley direction.
U.S. Pat. No. 6,862,141 discloses an optical substrate that features a three-dimensional surface having a correlation length of about 1 cm or less. The optical substrate is defined by a first surface structure function modulated by a second surface structure function, the first surface structure function producing at least one specular component from a first input beam of light. The peaks of the three-dimensional structure lie on the same plane. The optical substrate is suitable for use in a variety of applications, including brightness enhancement. This disclosure proposes a rather complicated method to derive the surface structure for the optical substrate. It is unclear from the disclosure how the optical substrate can actually be physically implemented. Further, it is doubtful of the level of brightness enhancement that can be achieved with the disclosed structure, as compared to prism films.
Most of the current backlight modules used in the flat panel display use two light enhancement sheets in order to have better output brightness. However, a troublesome optical coupling effect (i.e. wet-out) more frequently occurs when the two stacked light enhancement sheets get close to each other. The convention methods for reducing the occurrence of the wet-out let the two optical films physically away from each other as far as possible. That is, the heights of adjacent portions of the two optical films are accordingly modified. U.S. Pat. No. 5,771,328 disclosed an optical film having taller prisms (See numeral reference 56 in its drawings) and shorter prisms (See numeral reference 54 in its drawings). When the optical film is disposed on another one, the physical proximity to another is limited so that the visibility of a wet-out pattern is reduced. However, the taller prisms of the prior art still may be visual as straight lines through the optical film.
What is needed is a cost effective optical substrate that provides a surface structure that enhances brightness, avoids wet-out and reduces moiré effect in a single substrate.